1. Field of the Invention
The present invention relates to testing conditions and performing operations in well bores.
2. Description of Prior Art
Prior art well testing apparatus, as exemplified by U.S. Pat. Nos. 2,686,039; 2,689,920; 2,717,039; 2,814,019; 2,817,808; 2,869,072; 3,004,427; 3,006,186; 3,095,736; and 3,233,170, have been used to locate the freepoint, or location at which pipe or tubing was stuck, in a well bore. Several problems have existed in the prior art.
Accuracy of the readings obtained in freepoint sensing has been limited by the linearity of the response and the range of displacement of the freepoint sensor. Alignment or placement of the freepoint sensor at a proper null or reference was necessary before reliable readings were obtained. However, movement of the sensor through the well bore into a position for testing often moved the sensor out of proper alignment.
Additionally, when a back-off tool was used to loosen the stuck pipe in conjunction with freepoint sensing, further problems arose. Isolation between electrical circuits of the freepoint indicator and back-off tool, necessary from a safety standpoint, was often difficult to maintain. Further, the shock formed when the back-off tool was used to loosen pipe often damaged the relatively sensitive downhole electronic circuits in the freepoint indicator.
Further problems have arisen for these tools when used in recently drilled wells which generally extend to greater depths than prior wells. Heat at these greater depths significantly limited the operation of the electronics used in the well tools, particularly in the freepoint indicators. The increased length of wireline necessary to lower the tools to the greater depths has increased the electrical resistance of the wireline, requiring an increase in the electrical current sent from the surface to insure operation of the backoff tool, thus increasing the voltage drop along the wireline.